Research On Multi-dimensional Manipulations Of Random Fiber Lasers And Its Applications

Random fiber laser(RFL)based on random distributed feedback breaks through the conventional cognition of fiber lasers.Its complex physical mechanisms and open cavity structure provide a good platform for basic research and multi-dimensional manipulations of fiber lasers.Beneficial from its characteristics of modeless,stable output,simple structure and high conversion efficiency,random fiber lasers have experienced the transformation from the initial single form to multiple types after more than ten years of rapid development.The research depth and breadth of random fiber lasers have been greatly enriched from the in-depth research on the theoretical basis of spectral evolutions and time-domain characteristics,to realizations of broadband tunable,multi-wavelength,narrow-linewidth,high-power and low-noise fiber lasers,and then to the wavelength band expansion from near infrared to visible and mid infrared,as well as applications in fields of optical fiber sensing,communications and imaging.Compared with traditional fiber lasers,random fiber lasers have significant advantages in high power/high efficiency,broadband tunability and low noise,and become a new research direction of fiber lasers.This is still a development goal of random fiber lasers to use the above advantages to carry out multi-dimensional manipulations,such as time domain,spatial domain and spectral domain,achieve breakthrough in laser performance and expand its applications.In this dissertation,we focus on manipulations and applications of random fiber lasers based on Rayleigh scattering feedback in spatial,frequency and time domain.In the spatial domain,a multimode random fiber laser based on a master oscillator power amplifier is built to realize more than 100 W laser output with high spectral density and low spatial coherence,and is applied to speckle-free imaging.In the frequency domain,a broadband programmable spectral filter is designed and implemented based on the spatial-spectral mapping of a large-core multimode fiber,and is applied to Raman random fiber laser system to realize laser output with broadband tunability and arbitrary spectral shape.In the time domain,based on the low-noise characteristic and open cavity structure,random fiber lasers are used as pump sources in mode-locked Raman fiber lasers and optical rogue wave regulation,and mode-locked square-wave pulses and random pulses are both realized.The main research contents of this dissertation are summarized as follows:(1)A low-spatial-coherence high-power random fiber laser is realized,and specklefree imaging based on this light source is studied.Firstly,the output light field of multimode fiber is simulated by solving the generalized nonlinear Schr(?)dinger equation.It is found that the increase of input power can increase the number of effective modes and reduce the spatial coherence,which reveals the dependence of spatial coherence on the input power.Based on this result,a high-power random fiber laser system based on the main power oscillation amplifier is built,which combines the high spectral density of random fiber lasers with the decoherence characteristic of large-core diameter step-index multimode fibers and realizes a laser source with an output power of more than 100 W,a bandwidth of about 0.4 nm,and speckle contrast lower than the speckle perception threshold of human eyes.Result of speckle-free imaging illuminated by this light source shows that the realized image has a high contrast to noise ratio.This work provides a new light source for large field-of-view speckle-free imaging in high-loss applications.(2)A Raman random fiber laser with flexible spectrum is realized in combination of a segment of large-core diameter step-index multimode fiber and a spatial light modulator.Firstly,scattering characteristics of the multimode fiber are studied,including spatialspectral mapping and spectral correlation bandwidth.On this basis,a spatial light modulator is used to shape the wavefront of the multimode fiber,and a programmable spectral filter is realized using optimization algorithms,which has characteristics of wide tuning range and arbitrary defined spectrum filtering.The realized programmable spectral filter is introduced into Raman random fiber lasers as a point reflector.Beneficial from the broadband gain and feedback and open-cavity structure of Raman random fiber lasers,single-wavelength tunability lasing and definable multiwavelength lasing at 1.5 ?m and1 ?m are realized,respectively.In addition,different from the existing realization method of high-order tunable random fiber laser by adjusting the operating wavelength of the pump source,this work has realized a separate wavelength manipulation of each order lasing of the cascaded Raman random fiber laser using the programmable spectral filter while keeping the operating wavelength of the pump unchanged,which effectively reduces the spectral broadening and fully proves the broadband characteristic of the filter and its ability to manipulate spectrum of active fiber systems.The realized laser source has characteristics of flexible spectrum,wide bandwidth and compact structure,and has potential applications in fiber communication and sensing.(3)Both mode-locked square-wave pulse and optical rogue wave manipulation are realized under random fiber laser pumping.For mode-locked pulse generation,beneficial from the characteristics of high spectral density and low noise,random fiber laser is used as the pump source of mode-locked Raman fiber laser to realize noise-like square-wave pulses.Compared with traditional Raman fiber laser pumping,the achieved signal-tonoise ratio of pulses at fundamental frequency is 6 d B higher under random fiber laser pumping.The pulse width and repetition rate can be adjusted by pump power and intracavity polarization state of the ring cavity.For random pulse generation,the optical rogue wave can be realized by matching the wavelength of random fiber laser with the zero-dispersion wavelength of True Wave fiber and inducing nonlinear effects using random fiber laser.Furthermore,the pulse of optical rogue wave is changed from disorder to quasi order by actively modulating the pump source.The realized noise-like pulse and random pulse light sources have good application potential in the fields of information coding,lidar and super-resolution imaging.